IS-IS Shortcuts

MPLS traffic engineering maps certain data flows to established label-switched paths (LSPs) rather than to data links calculated by the interior gateway protocol (IGP) to be part of the best (shortest) path. Fundamental to this function is the determination of what traffic is to be mapped to an LSP. Traffic is mapped to an LSP at the tunnel's ingress label-switching router (LSR) by designating the egress LSR as the next-hop router for certain destination prefixes. It is important to understand that the LSP does not constitute an entire route to a destination. Rather, the LSP is a next-hop segment of the route. Therefore, packets can be mapped to an LSP only if the egress LSR is considered to be a feasible next-hop candidate during the route resolution process. Interior gateway protocol (IGP) shortcuts, also called traffic-engineering shortcuts, provide a tool by which the link-state IGP (for example, IS-IS) in an autonomous system can consider an LSP in its shortest-path-first (SPF) calculations.